A federal judge in Milwaukee struck down Wisconsin's voter identification law on Tuesday, saying a requirement that voters show a state-issued photo ID at the polls imposes an unfair burden on poor and minority voters.

US district judge Lynn Adelman sided with opponents of the law, who argued that low-income and minority voters aren't as likely to have photo IDs or the documents needed to get them. Adelman said the law violated the US constitution's guarantee of equal protection.

Adelman's decision invalidates Wisconsin's law and could set a precedent for similar legal challenges in Texas, North Carolina and elsewhere. At least 14 states require voters to show photo ID, and legislation in dozens of other states includes proposals to either introduce new voter ID laws or strengthen existing ones. Last week, an Arkansas judge struck down that state's voter ID law; the decision is being appealed to the Arkansas supreme court.

Wisconsin's Republican-led legislature passed the photo ID requirement in 2011, scoring a long-sought GOP priority. Former Governor Jim Doyle, a Democrat, had vetoed a similar requirement three times between 2002 and 2005.

Republican backers argued that requiring voters to show ID would cut down on voter fraud and boost public confidence in the integrity of the election process. Democrats countered that Republicans never produced evidence of any widespread fraud in Wisconsin, and said the mandate would keep poor people, immigrants and senior citizens from voting.

Wisconsin's law was only in effect for a 2012 primary, before a Dane County judge declared it unconstitutional.

The federal challenge combined two cases. One was brought by minority-rights groups, including the Wisconsin chapter of the League of United Latin American Citizens, and the other involved the American Civil Liberties Union.

The plaintiffs argued that the law violates the federal Voter Rights Act, but their case wasn't the only legal challenge. In state court, the League of Women Voters and the National Association for the Advancement of Colored People's Milwaukee branch filed separate lawsuits alleging the law violates Wisconsin's constitution.

Those challenges are being considered by the Wisconsin supreme court, which heard arguments in late February; it is not clear when the justices will rule.

For the voter ID law to be reinstated, it will have to pass both the federal and state challenges. However, Adelman's decision does not necessarily render the state cases moot, because his decision likely will be appealed.